The present invention relates to a servo friction brake of the type as disclosed in European patent application No. 0 195 922. The special feature of the prior art friction brake is that a valve for sensing wheel lock is interposed between the two friction linings. As soon as the wheel associated with the respective brake locks, the valve will generate hydraulic pressure which is used to actuate a second valve that permits change-over of effective surfaces provided within the wheel brake cylinder. The result is a reduction of the actuating force applied to the linings so that an anti-lock effect is ensured this way.
It is a shortcoming in the prior art friction brake that hydraulic intervention in the wheel brake cylinder becomes effective only in a case of wheel lock. The intervention includes a simple change-over action so that the brake of the art exhibits a high degree of friction coefficient sensitivity despite the intervention.
Therefore, an object of the present invention is to provide measures permitting a major reduction of the friction coefficient sensitivity. An additional objective is to achieve a great servo effect, especially in the range close to the wheel lock limit.